Force-triggered applanation tonometer

ABSTRACT

A portion of the cornea or similar flexible membrane is flattened by urging against the cornea a footplate with a measurable force and deriving a signal proportional to the magnitude of the applanated area of the cornea when the applanating force is compared to a threshold which corresponds to a force equal to a selected reference force on the planar footplate contacting the cornea. When the selected reference force signal is reached the area of the applanated region of the cornea is transferred as a signal for scaling by another signal proportional to the force.

FIELD OF THE INVENTION

The invention is in the field of tonometry and particularly relates toautomated apparatus for opthamalogical measurements suitable for usewithout anaesthetic.

CROSS-REFERENCE TO RELATED APPLICATION

Copending U.S. Ser. No. 599,968, commonly filed herewith, describesother related inventions closely related to the present invention.

BACKGROUND OF THE INVENTION

The measurement of the intraocular hydrostatic pressure is a majordiagnostic tool for the identification of eye disorders, especiallyglaucoma, and as such, apparatus capable of accurate and reliablemeasurement is greatly to be desired. Tonometry is used routinely forscreening the population at large as well as following individuals withknown pathology: consequently, risk, inconvenience and trauma must beminimized. Moreover, of those tested, the overwhelming majority will notexhibit intraocular pressure pathology. An accurate and reliablemeasurement is essential to assure that the screening does indeedidentify incidence of abnormal intraocular pressure while not burdeningthe health care system with erroneous positive identification ofpathology in normal individuals. Other desiderata include safety,non-invasiveness and practice of the method without the necessity oftopical anaesthetic.

In the prior art, the Goldman tonometer has been a standard foropthamalogical measurement for many years. In this approach theintraocular pressure is obtained by flattening a standard area of thecornea to conform to a planar surface placed in contact with the cornea.Applanation tonometry, as the method is known, employs a circulartransparent plane surface of precisely known diameter which is urgedagainst the anaesthetized cornea while the observer confirms theapplanation condition by observing the cornea through the transparentplane surface or footplate with the aid of a small amount of flouresceinin the lacrimal fluid and a slit lamp or smilar light source. Theoptical source is preferably rich in the blue portion of the spectrum toexcite the fluorescein and thereby provide enhanced contrast. Theobserver adjusts the pressure applied to the foot plate until the corneajust conforms to a circular region marked on the transparent foot plate,at which point the force urging the foot plate against the cornea isrecorded.

Measurements of this type suffer from error in establishing theapplanation condition. This determination is subjective and prone toerror arising from, among other effects, the presence of a meniscus oftear fluid at the periphery of the foot plate and the resistance of thecornea to bending. A significant source of error and difficulty ariseswith the length of time required to adjust the device and to observe theflattened area. This may require an interval ranging from a few secondsto a minute or more. It is difficult for the subject to maintain the eyein a fixed position for that period without blinking. Moreover, suchprotracted contact of the instrument with the cornea increases the riskof a scratch or other trauma because of the prolonged contact and thepossiblity of gross eye movements. Clearly, the prolonged contact alsorequires application of an anaesthetic to the eye.

A significant improvement in applanation tonometry apparatus due toMackay and Marg (see Marg et al, Archiv. Opthalm., v.4, no.1, pp 67-74(1961) and references therein) utilizes electronic means to measure theforce required to produce the applanation condition between the footplate and the cornea, deriving a signal proportional to the appliedforce. The force is necessarily applied as a function of time and theresulting displacement of a plunger linking the cornea with the centralregion of the foot plate is monitored on a trace recorder. In theMackay-Marg instrument the trace characteristically rises to a firstrelative maximum as the central plunger responds to the full cornealresistance and the intraocular pressure. As the cornea is applanatedagainst the footplate region surrounding the central plunger, thecorneal resistance is distributed over the annular region and the signalderived from the plunger displacement drops to a relative minimum,thereafter rising monotonically as the cornea continues to yield inresponse to the increasing pressure. A second maximum will be recordedwhen the area of the cornea applanated by the probe reaches its maximum.As the probe is withdrawn, the sequence is reversed and a near mirrorimage of the trace is generated. For this type instrument it has beenfound that the significant indicia for establishing the magnitude of theintraocular pressure is the relative amplitude of the aforementionedrelative minimum or trough with respect to the baseline of the trace.The force measurement is derived from the displacement of the plunger.The displacement is quite small, of the order of a micron. TheMackay-Marg tonometer is (at least in principle) free of the need for ananaesthetic because the entire measurement is obtained in an interval ofthe order of 10's of milliseconds. An interpretation of the complextrace is still required for this instrument to extract the criticalintraocular pressure parameter.

BRIEF SUMMARY OF THE INVENTION

It is an object of the invention to provide an improved tonometerespecially exhibiting improved consistency in measurement.

It is another object to simplify tonometrical instrumentation and toreduce the influence of subjective judgement in both acquisition andinterpretation of the data.

In one feature of the invention a selected force is applied to a planarprobe in contact with the cornea.

In another feature of the invention means are provided to electronicallymonitor the magnitude of the applanation area.

In yet another feature of the invention comparator means are provided totrigger the measurement of the instantaneous applanation area uponsensing equality to, or an excess thereof over a selected thresholdvalue of the applanation force.

In still yet another feature of the invention scaling means are providedto compute and display the pressure obtained from the measured area.

In again another feature of the invention, the applanation area iscontinuously sensed by measurement of the capacitive reactance from theplanar probe to ground potential as referenced from the cornea.

In one simple embodiment a linear variable differential transformer(LVDT) is disposed to sense the applied force through theforce-displacement relationship of the springs which support a shaftcoupling the LVDT to the footplate of the instrument. Such devices arewell known and have been employed in similar fashion in the art. Thefoot plate of the tonometer transmits the applied force through theshaft from the cornea to the armature of the LVDT. The force sensed fromthe output fo the LVDT (or other alternative device for obtaining aforce proportional signal) is compared with a reference in a comparatorand when equality with the reference is sensed a trigger signal isgenerated. An insulative sheet of known dielectric constant and geometryprovides D.C. isolation between the cornea and the foot plate to whichan A.C. signal is applied. The A.C. current to ground (the cornea) ismeasured to ascertain the capacity which in turn is determined by theapplanation area. Upon the triggering signal from the force signalcomparator, the A.C. current for a constant voltage (or the A.C. voltagefor constant current) is sampled and presented to an amplifier to yieldan area proportional signal.

The foregoing objectives, features and advantages of the presentinvention will appear from the following more particular description asillustrated in the accompanying drawings.

BRIEF DESCRIPTIONS OF THE DRAWINGS

FIG. 1a is a schematic explanation of an embodiment of the presentinvention.

FIG. 1b is a schematic representation of a mechanical suspension.

FIG. 2a is a partial section of a a probe assembly and associated basicsignal processing.

FIG. 2b is an alternate electrical connection structure for thefootplate signal.

FIG. 3 is a symbolic diagram of another type of signal processing.

FIG. 4 represents a piezo-electric transducer for the footplate of thepresent invention.

FIG. 5a shows another embodiment for a variable reluctance forceproportional transducer, and FIG. 5b illustrates the basic circuittherefore.

FIG. 6 shows a constant applanation force-referenced tonometer.

DETAILED DESCRIPTION OF THE INVENTION

One simple embodiment is shown schematically in FIG. 1. Ashaft 30supports a foot plate 32 which bears against the cornear 34 for aninternal pressure measurement. The foot plate is electrically isolatedfrom cornea 34 by dielectric 36 and the shaft is isolated from ground byappropriate means. The force exerted by the cornea against the footplate is balanced by springs 40. The relative displacement of the shaft30 is ascertainable by means of a linear displacement sensor 37 fromwhich a signal is taken and which is calibrated to yield a forceproportional signal in accord with

    F=-k x

where k is the spring constant for the springs 40 and x is the relativedisplacement of the shaft 30.

An AC signal 103 is presented to a high gain operational amplifier 102which is connected in such manner that the signal applied to footplate32 exaclty duplicates the AC signal 103. The voltage appearing on thefootplate is returned to the inverting input 202 of the operationalamplifier 102. A difference appearing between the excitation signal 103and the footplate signal will be greatly amplified and will appear atthe output. The amplified output is, in turn, connected to the footplatethrough series impedance 101 of magnitude Z. Provided that certainstability requirements are met and that the gain of operationalamplifier 102 is sufficiently large (of the order of 10³ to 10⁵), thefootplate potential will be a very close reproduction of the excitationsignal 103. Any current flowing from the operational amaplifier outputto footplate 32 will necessarily flow through series impedance 101thereby imposing a potential difference between the inputs of thedifferential amplifier 104. The voltage difference will be proportionalto the current flowing to the footplate 32 and proportional to theimpedance Z.

The footplate 32, shaft 30, springs 40, and associated itemselectrically connected to the footplate are guarded by a shield 107driven to the same potential as the footplate. Current flowing into thefootplate results from the capacitive coupling to the cornea. Thecapicitive coupling is clearly proportional to the contact area betweenthe cornea and the footplate. The surface of the cornea is relativelygood conductor and is maintained at ground potential through theelectrical contact of the patient with his environment or by therelatively large capacitive to ground presented by the human body evenwithout direct ohmic contact to ground. If the impendance 101 iscapacitative, the voltage developed a cross it will be in phase with theexcitation 103 because the footplate current will lead the phase ofexcitation signal 103 by 90°. This current will in turn produce a dropacross the capacitor that lags the phase of the current by 90°, therebyproducing a resulting voltage signal at the differential amplifier thatis in phase with the excitation signal 103. It is apparent that theimpedance 101 need not be a capacitative reactance: it is only necessaryto note that the voltage developed across it and the phase relationshipfor that voltage are selectable by the designer. The detector 106 ispreferably a phase sensitive circuit in order to exploit synchronousproperties and enhance noise rejection. It is important to note thatphase sensitive detector is not essential for this application and apeak detector, envelope detector or similar means for producing a dcsignal from the ac signal across impedance 101 would be suitable(although somewhat less satisfactory) for the purpose.

One suitable mechanical suspension for support of shaft 30 isillustrated in FIG. 1b which is a schematic perspective illustration ofa flat spring suspension structure. Flat springs 40 are formed byetching a metal foil. Arc segment perforations, as shown are found toprovide enhanced radial compliance and a wide dynamic range. Springcarriers 58 are secured to probe housing 107 and the springs 40 are heldin the respective carrier, against the end plate thereof by a metal ring59 press fit into the carrier ends.

An A.C. signal of constant amplitude is applied to shaft 30 and a highinput impedance amplifier receives the signal to ground through thecapacitive impedance presented by dielectric 36. The signal from theimpedance divider is proportional to the impedance presented by thecapacitive coupling to the cornea.

In operation the force proportional signal is adjusted to a selectedreference value through a comparator and a trigger signal is derivedwhenever the force proportional signal equals the selected valuepresented as reference input to the comparator.

The response of the present apparatus is noted to produce signals whichare monotonic functions of applanation in contrast to the complex signalobtained from the prior art instrument of Mackay and Marg.

It is recognized in the field of tonometry that the cornea is notperfectly flexible and the finite rigidity of the cornea provides anapparent increment to the measured pressure. It is also known thatsurface tension forces operate between the lacrimal fluid and theexterior corneal surface to reduce the applied force required for agiven deformation. These two effects are oppositely directed and it hasbeen determined that for the standard applanated areaof prior art (3.06mm diameter) the two effects are of approximately equal magnitude: thus,for this standard area, the pressure derived from independent area andforce sensors need not be corrected for these two effects (within theaccuracy of their cancellation). In copending U.S. Ser. No. 599,968, astandard area is directly selectable through a proportional signal foroperation directly comparable with prior art, if so desired. The presentinvention describes an instrument properly regarded as referencingmeasurements to a standard applanating force.

As above described, one embodiment of the subject tonometer monitors oneparameter (area) in a substantially continuous manner and for a criticalpreselected magnitude of the applanation area there is developed atrigger signal for obtaining a signal proportional to the applied force.It is recognized that the response of the cornea or other deformablemembrane to the applanation may be continuously monitored in both thearea signal and the force signal to establish the continuous,two-dimensional response function rather than a particular discretepoint on that function. One thereby obtains access to a wealth ofinformation latent in the shape of the response function. Cornealrigidity, hysteresis, corneal bending and like quantifiable parametersare thereby accessible to measurement and study. It is noted that thechoice of foot plate area was selected in the prior art to substantiallyminimize a corneal rigidity effect. One may well wish to measure theeffect of corneal rigidity and other attributes which may contribute toeffective diagnoses. The localization of regions of this generalizedtwo-dimensional function space for study is accomplished instraightforward fashion by constraint of signals or by constraintsimposed upon the recorded two-dimensional data.

A more detailed exposition of a preferred clinical embodiment withreference to corneal tonometry is shown in FIG. 2a. A probe case 50 ofcylindrical symmetry contains LVDT windings 56 which effectuate sensingthe displacement of the LVDT core 54. The LVDT comprises three windings:a driven primary and two symmetrically situated secondaries, connectedin series opposed form. The flux arising from the driven primary linksthe core and the two secondaries. With the core 54 at zero displacement,symmetrically disposed with respect to the secondaries, equal opposedvoltages are induced across the secondaries for a net null signal. Upondisplacement of the core 54, the voltages across the secondaries becomeunbalanced and a difference signal obtained from the LVDT exhibits phaseand amplitude dependent upon the direction and magnitude of thedisplacement. This signal is then processed to yield a waveformfaithfully reproducing the motion of the core.

The LVDT core 54 is supported near one end of shaft 30 with foot plate32 at the other end. Hypodermic syringe stock is recommened as anexcellent available stock for shaft construction. A thin insulating filmconstitutes the dielectric of a capacitive coupling between foot plate32 and the cornea. The insulating film 36 may be a polymeric coating, aglass or fused silicon dioxide. Polyurethane, mylar, polyethylene,polyester, epoxy, acrylic and the like are all very good examples forthis purpose because these substances exhibit relatively low toxicicityand because they exhibit high dielectric constants. Glass or silicondioxide films have the advantage of superior chemical and dimensionalstability as well as damage resistance owing to their hardness. Certainother materials, such as anodized aluminum, are also suitable.

A spring carrier 58 is mechanicaly secured to the probe case 50 tosupport the shaft 30 via support springs 40 and to provide electricalcoupling thereto. An apprpriate spring suspension which has beenemployed for this application is shown in FIG. 2b wherein a metal foilis etched to remove annular segments as shown in FIG. 1b. The resultingflat springs are secured to the spring carrier by a press fit ring. Theshaft 30 is secured to the central hole by bonds 60 formed from knownconducting epoxy resin.

Capactive coupling to the cornea through foot plate 32 and dielectricfilm 36 results in an area proportional signal if spurious currentsthrough stray capacitances can be eliminated or compensated. For thispurpose, the probe structure incorporates a guard conductor shell 62surrounding the foot plate 32, shaft 30 and spring suspension.Insulating shell 64 isolates the guard conductor 62 from the probe case50 and insulator 66 likewise isolates the spring carrier 58 from guardconductor 62.

A preamplifier 68 preferably housed inside of guard conductor 62comprises a differential ampifier 70 for comparison of the foot platesignal separately from the parasitic currents arising from the straycapacitances.

A preferred structural variation of the above described probe is shownin FIG. 2b. The footplate 32 is here joined mechanically to hollow shaft30 through insulated collar 31. An electrical coupling from footplate 32to preamplifier 70 is realized from an insulated conductor 69 which iscarried coaxially in hollow shaft 30. The springs 40 are aloselectrically isolated from the shaft 30 and the later is, in thisvariation, driven to guard potential.

It is useful at this point to consider the amplitude of the desiredcapacitive current resulting from contact between the cornea and footplate 36. Consider a representative thickness of 0.001 inch (25.4microns) and a relative dielectric constant of 3.6 for the dielectricfilm 36. Glasses, and in particular fused silicon dioxide exhibitdielectric constants in this range and many common polymeric coatingshave similar dielectric constants.

Oscillator 72 provides an AC excitation at a frequency which for presentpurposes can be assmed as 10 KHz. Under the assumption of a 10 volt peaksignal the AC current through the foot plate 32 is very nearly 6microamperes. While this is not difficult to measure directly, theeffects of parasitic capacitance (which can induce currents that reachor exceed this value) are effectively removed by floating the guardshell conductor and the entire preamplifier to foot plate potential. Thesignal is amplified to the point where interwinding capacitanceintroduces neligible effects, at which point the signal is returned toground through transformer coupling 74. This area proportional signal isagain amplified by linear amplifier 76, phase detected against theoscillator reference signal in sync detector 77 from which there isobtained a DC signal representative of the applanation area. In the samefashion, the LVDT excitation is amplified by amplifier 78 and phasedetected against the oscillator reference at sync detector 79 to yield aforce proportional DC signal.

A more specialized apparatus may be obtained following FIG. 3 whereinthe force proportional signal is compared with a reference level to gatean area proportional datum to a processor to yield a pressure variablefor display. A preferred addition to this arrangement is an electricalcontact means responsive to the motion of the shaft 30, in turndependent upon the applanating force. In this manner threshold sensingtriggered from a reference magnitude force would be acheived in aparticularly simple manner.

It is noted that further processing of the measured proportional signal,if desired, is symbolically contained within a logic unit (not shown).An example of such optional processing would be an averaged sampling ofthe sensor transient on both the rising and falling sides, correspondingto rise and fall of the applanation condition. (There are technicalreasons which tend to reduce the value of sampling on the fallingportion of the transient for simple intraocular pressure measurement.)The logic unit processes the signal in accordance with the relationshipof the derived parameter (intraocular pressure, for example) to theforce response transient, which in the present apparatus contrasts withthe transient waveform of the McKay-Marg instrument. The work of Mackayand Marg suggest that the intraocular pressure is proportional to theamplitude of the relative minimum of the transient force responsewaveform. The operational principle underlying the transient waveform ofthe present apparatus yields a monotonic function, which when scaled ata selected area magnitude accurately measures the intra-ocular pressure.Further optional processing, already alluded to herein, includesmultiple sampling at known succesive values of the area proportionalsignal to yield a two parameter analysis of the corneal behaviour. Thedetails of this aspect of the processing are outside the scope of thepresent invention.

One will readily appreciate that alternative pressure sensing means canbe employed in the form of a piezo-electric transducer for directsensing of the applied force. A piezoelectric transducer is conceptuallyillustrated in FIG. 4 wherein a footplate 32 is supported as in otherembodiments by shaft 30. The distal end of shaft 30 is bonded to abimorph piezoelectric element 300. The latter typically comprises aconductor such as an aluminum disk 301 bonded to a peizoelectric crystal302. The force applied to the footplate in contact with the cornea istransmitted through shaft 30 to the bimorph disk 300 causing the latterto assume a slightly concave shape, thereby inducing radial tensilestresses in the peizo crystal. A potential developed between the planesurfaces of the bimorph sensor 300 is proportional to the transientapplied force and the resulting voltage pulse is directed to a highimpedance amplifier 303. A pulse with amplitude proportional to theapplied force is thereby obtained for use in processing as in the abovedescribed embodiments.

Another transducer for obtaining a force proportional signal isillustrated in FIGS. 5a and 5b. This variable reluctance sensor isstructuraly similar to the LVDT with the distinction that no ACexcitation, AC amplifier nor synchronous detection are employed. Asdistinct from the LVDT apparatus, which develops a signal proportionalto the absolute displacement of the LVDT core, the variable reluctancesensor yields a signal proportional to dz/dt, the rate of displacementalong the z axis of a permanent magnet 87 with respect to windings 88aand 88b. In principle this signal can be integrated by integrator 89 toyield the z displacement. Sufficient integration is inherent in an ACamplifier exhibiting aapproximately -20 db/decade rolloff over theappropriate frequency range (about 1 to 100 Hz). The resulting pulseamplitude is therefore force proportional through the displacementproportionality and may be treated as in the above embodiments.

In the above described embodiments, electrical measurements of theapplanated area proportional signal and applanation force proportionalsignal are combined to yield the desired pressure proportional signal.Another approach, further described in the aforesaid copending U.S. Ser.No. 599,968, utilizes an area proportional signal as before, but theprobe now comprises a structure supported directly on the cornea of asupine patient, as schematically illustrated in FIG. 6. The applanationof the cornea under gravity against the footplate 32 defines a standardreference value (which is not, as in the present invention, simplyadjusted by varying a reference level input to a comparator) and it isonly necessary to measure the resulting area proportional signal asdiscussed herein to obtain a signal which transforms to a pressureindicia. It is recognized that both the constant (reference) force andthe force triggered tonometer (the present invention) differ fromstandard practice in tonometry in the precise prinicple of a fixedapplanating force and variable degree of applanation whereas standardpractice as well as above described embodiments emphasize a fixedreference applanation area and variable applanation force.

It will be apparent that many changes could be made in the above methodand apparatus and many apparently different embodiments of thisinvention could be made without departing from the scope thereof; it istherefore intended that all matter contained in the above descriptionand shown in the accompanying drawings shall be interpreted asillustrative and not in a limiting sense.

What is claimed is:
 1. An applanation tonometer for ascertaining theinternal hydrostatic pressure of a flexible body containing a fluid,said body subject to curvature due to said internal hydrostaticpressure, comprising(a) a footplate comprising a planar portion forbearing against said flexible body, a shaft for supporting saidfootplate and a guard electrode spaced apart from said footplate andsubstantially surrounding said footplate, (b) housing means forsupporting said footplate in a housing, said housing means capable ofdisplacement toward said body whereby a portion of the surface of saidflexible body is flattened against against said footplate, (c) forcesensing means for ascertaining the magnitude of the force exerted bysaid body against said footplate and for generating a force proportionalsignal, (d) area signal generating means for developing a signalproportional to the area of said flexible body in contact with saidfootplate comprising oscilator means for driving both said footplate andsaid guard electrode to the same a.c. potential, (e) comparator meansresponsive to said force proportional signal means for establishing theequality or excess of said force proportional signal with respect to athreshold value of selectable magnitude for generating a trigger signal,(f) scaling means responsive to said trigger signal for operating on theinstantaneous signal magnitude of said area proportional signal with aquantity dependent upon said force proportional signal to obtain apressure value, and (g) display means to indicate said pressure value.2. The applanation tonometer of claim 1 wherein said housing meansfurther comprises spring means for coupling said shaft to said housingmeans.
 3. The applanation tonometer of claim 2 wherein said area signalgenerating means comprises means for measuring the capacitive reactanceof said footplate in series with said flexible body.
 4. The applanationtonometer of claim 2 wherein said force sensing means comprises a linearvariable differential transformer mechanically coupled to said shaft. 5.The applanation tonometer of claim 2 wherein said force sensing meanscomprises a piezo electric transducer mechanically coupled to saidshaft.
 6. The applanation tonometer of claim 2 wherein said forcesensing means comprises variable reluctance transducer means to generatea signal responsive to the displacement of said footplate.
 7. The methodof measuring the internal hydrostatic pressure within a flexible bodycontaining a fluid, said body bounded by walls, said walls outwardlycurved due to said hydrostatic pressure, comprising the steps of(a)applying a force against a portion of said outwardly curved walls todeform a portion thereof to conform to a planar surface portion, (b)sensing the area of said planar portion of the surface of said flexiblebody, comprising the steps of coupling an oscillatory current throughsaid planar aspect of the deformed body while maintaining anequipotential surface substantially about said oscillatory current inthe neighborhood of said planar aspect by applying an oscilatingpotential to said surface, said oscillating potential corresponding tosaid oscillating current in phase and substantially equal to thepotential at said planar aspect, (c) generating a trigger signal upondetecting a preselected relative magnitude of said force proportionalsignal with a selected value, (d) measuring the magnitude of said areaapplanated upon the detecting of said preselected relative magnitude,(e) computing the ratio of said applied force to said area, and (f)displaying said ratio.
 8. In corneal applanation tonometer,(a) footplatemeans for applanating the subject cornea, comprising an applanating diskhaving a dielectric layer disposed on one side thereof, and footplatesupport means for supporting said footplate means in contact with thecornea whereby a portion of said cornea is flattened against saidfootplate means, (b) guard electrode means substantially surroundingsaid footplate means and spaced apart therefrom for providing acontrolled equipotential surface thereabout, (c) oscillator meansproviding an oscillating current coupled to said guard electrode meansthrough a first impedance and coupled to said footplate means through asecond impedance and thence through said dielectric layer to the cornea,(d) differential amplifier means having respective inputs coupled tosaid footplate means and said guard electrode means for obtaining adifference signal proportional to the oscillating potential differencebetween said footplate means and said guard electrode means, (e) phasesensitive detector means for developing a d.c. signal from the phaserelated difference signal and oscillator means, whereby said d.c. signalrepresents the capacitive impedance coupling of said corneal and saidfootplate means and thus said d.c. signal comprising an areaproportional signal dependent upon said flattened area of said cornea,and (f) force sensing means for obtaining a force proportional signaldepending in magnitude upon the force applied by the cornea against saidfootplate means, (g) discriminator means for comparing said forceproportional signal against a reference level and deriving therefrom atrigger signal for gating said area proportional signal.
 9. Theapparatus of claim 8 comprising display means for representing saidgated area proportional signal.